In a 2011 interview, The Daily Bell asked Joe Salerno about Hans Sennholz. Here was his answer:
Daily Bell: Why do you think so highly of Hans Sennholz? Why was he under-appreciated?
Dr. Joseph Salerno: I greatly admire the work of the late Hans Sennholz because he embodied so well the virtues and skills of a vocational economist. Sennholz did his Ph.D under Mises and attended Mises’s NYU seminar in the company of prominent Austrians such as Rothbard, Kirzner, and Hazlitt, among others. He was therefore intimately acquainted with causal-realist economic theory. During his long and illustrious teaching career and in his voluminous writings he aptly employed this theory to elucidate and analyze the causes and solutions of the economic problems of the day as well as the dire consequences of all manner of interventionist economic policies, which have been the outgrowth of the modern American Welfare/Warfare State.
Not only did he educate and enlighten generations of students and lay readers, Sennholz was also an accomplished scholar and innovative thinker who made important contributions to economic science and the sound money program. He published articles contrasting the Austrian with the Chicago (monetarist) theory of money and explaining the operation of the classical gold standard and why its restoration was the only alternative to inflation. These and other contributions on the theory and history of money and business cycles are integrated in his book Age of Inflation. He also wrote valuable scholarly essays on Menger’s monetary theory and Böhm-Bawerk’s capital and interest theory. Perhaps his most important publication, however, is his book Money and Freedom, which has been neglected even by hard-money Austrians. In this slim volume of less than 100 pages, he presented an innovative and eminently practicable transition plan to end the Fed’s monopoly of the money supply and to introduce an inflation-proof regime of monetary freedom.
In answer to the second part of your question, Hans Sennholz has been ignored by the mainstream, and even by some who call themselves Austrians, for two reasons. First, he wrote in plain English and did not mince words. As in the case of the great 19th-century French Liberal economist Frédéric Bastiat, clarity of writing style was mistaken for superficiality of thought. Second, Sennholz invariably focused on real-world problems and policies and never wasted a word analyzing contrived paradoxes and puzzles or arcane historical minutiae. Furthermore, he consistently applied the tried and true causal-realist method in his analysis and avoided the technique-driven fashions of game theory, experimental economics, behavioral economics, computable general equilibrium models, etc.